I always like to have an overflow rated for more flow then my pump.
But you have to take into consideration for head pressure on your pump.
So a 900 g/h pump only pumps only pumps 900 at 0' above the pump.
at 5 feet it might only pump 400 g/h
I used this calculator
http://www.reefcentral.com/calc/hlc2.php
to help me when I was setting my system up.
SR has a good point to you can do what he is saying but some pumps are not made to have a lot of back pressure. This can lead to a shortened life of your pump.
there is another way to do this by putting a pipe T and have a valve on the end of the tee which puts water back into the sump. By diverting water to the sump it does not hurt the pump and decrease the water to the tank.
Hope this helps
Todd